Protein Kinases
Kinases regulate many different cell proliferation, differentiation, and signaling processes by adding phosphate groups to proteins. Uncontrolled signaling has been implicated in a variety of disease conditions including inflammation, cancer, arteriosclerosis, and psoriasis. Reversible protein phosphorylation is the main strategy for controlling activities of eukaryotic cells. It is estimated that more than 1000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. The high energy phosphate, which drives activation, is generally transferred from adenosine triphosphate molecules (ATP) to a particular protein by protein kinases and removed from that protein by protein phosphatases. Phosphorylation occurs in response to extracellular signals (hormones, neurotransmitters, growth and differentiation factors, etc), cell cycle checkpoints, and environmental or nutritional stresses and is roughly analogous to turning on a molecular switch. When the switch goes on, the appropriate protein kinase activates a metabolic enzyme, regulatory protein, receptor, cytoskeletal protein, ion channel or pump, or transcription factor.
The kinases comprise the largest known protein group, a superfamily of enzymes with widely varied functions and specificities. They are usually named after their substrate, their regulatory molecules, or some aspect of a mutant phenotype. With regard to substrates, the protein kinases may be roughly divided into two groups; those that phosphorylate tyrosine residues (protein tyrosine kinases, PTK) and those that phosphorylate serine or threonine residues (serine/threonine kinases, STK). A few protein kinases have dual specificity and phosphorylate threonine and tyrosine residues. Almost all kinases contain a similar 250–300 amino acid catalytic domain. The N-terminal domain, which contains subdomains I–IV, generally folds into a two-lobed structure, which binds and orients the ATP (or GTP) donor molecule. The larger C terminal lobe, which contains subdomains VI A-XI, binds the protein substrate and carries out the transfer of the gamma phosphate from ATP to the hydroxyl group of a serine, threonine, or tyrosine residue. Subdomain V spans the two lobes.
The kinases may be categorized into families by the different amino acid sequences (generally between 5 and 100 residues) located on either side of, or inserted into loops of, the kinase domain. These added amino acid sequences allow the regulation of each kinase as it recognizes and interacts with its target protein. The primary structure of the kinase domains is conserved and can be further subdivided into 11 subdomains. Each of the 11 subdomains contains specific residues and motifs or patterns of amino acids that are characteristic of that subdomain and are highly conserved (Hardie, G. and Hanks, S. (1995) The Protein Kinase Facts Books, Vol I:7–20 Academic Press, San Diego, Calif.).
The second messenger dependent protein kinases primarily mediate the effects of second messengers such as cyclic AMP (cAMP), cyclic GMP, inositol triphosphate, phosphatidylinositol, 3,4,5-triphosphate, cyclic-ADPribose, arachidonic acid, diacylglycerol and calcium-calmodulin. The cyclic-AMP dependent protein kinases (PKA) are important members of the STK family. Cyclic-AMP is an intracellular mediator of hormone action in all prokaryotic and animal cells that have been studied. Such hormone-induced cellular responses include thyroid hormone secretion, cortisol secretion, progesterone secretion, glycogen breakdown, bone resorption, and regulation of heart rate and force of heart muscle contraction. PKA is found in all animal cells and is thought to account for the effects of cyclic-AMP in most of these cells. Altered PKA expression is implicated in a variety of disorders and diseases including cancer, thyroid disorders, diabetes, atherosclerosis, and cardiovascular disease (Isselbacher, K. J. et al. (1994) Harrison's Principles of Internal Medicine, McGraw-Hill, New York, N.Y., pp. 416–431, 1887).
Calcium-calmodulin (CaM) dependent protein kinases are also members of STK family. Calmodulin is a calcium receptor that mediates many calcium regulated processes by binding to target proteins in response to the binding of calcium. The principle target protein in these processes is CaM dependent protein kinases. CaM-kinases are involved in regulation of smooth muscle contraction (MLC kinase), glycogen breakdown (phosphorylase kinase), and neurotransmission (CaM kinase I and CaM kinase II). CaM kinase I phosphorylates a variety of substrates including the neurotransmitter related proteins synapsin I and II, the gene transcription regulator, CREB, and the cystic fibrosis conductance regulator protein, CFTR (Haribabu, B. et al. (1995) EMBO Journal 14:3679–86). CaM II kinase also phosphorylates synapsin at different sites, and controls the synthesis of catecholamines in the brain through phosphorylation and activation of tyrosine hydroxylase. Many of the CaM kinases are activated by phosphorylation in addition to binding to CaM. The kinase may autophosphorylate itself, or be phosphorylated by another kinase as part of a “kinase cascade”.
Another ligand-activated protein kinase is 5′-AMP-activated protein kinase (AMPK) (Gao, G. et al. (1996) J. Biol. Chem. 15:8675–81). Mammalian AMPK is a regulator of fatty acid and sterol synthesis through phosphorylation of the enzymes acetyl-CoA carboxylase and hydroxymethylglutaryl-CoA reductase and mediates responses of these pathways to cellular stresses such as heat shock and depletion of glucose and ATP. AMPK is a heterotrimeric complex comprised of a catalytic alpha subunit and two non-catalytic beta and gamma subunits that are believed to regulate the activity of the alpha subunit. Subunits of AMPK have a much wider distribution in non-lipogenic tissues such as brain, heart, spleen, and lung than expected. This distribution suggests that its role may extend beyond regulation of lipid metabolism alone.
The mitogen-activated protein kinases (MAP) are also members of the STK family. MAP kinases also regulate intracellular signaling pathways. They mediate signal transduction from the cell surface to the nucleus via phosphorylation cascades. Several subgroups have been identified, and each manifests different substrate specificities and responds to distinct extracellular stimuli (Egan, S. E. and Weinberg, R. A. (1993) Nature 365:781–783). MAP kinase signaling pathways are present in mammalian cells as well as in yeast. The extracellular stimuli that activate mammalian pathways include epidermal growth factor (EGF), ultraviolet light, hyperosmolar medium, heat shock, endotoxic lipopolysaccharide (LPS), and pro-inflammatory cytokines such as tumor necrosis factor (TNF) and interleukin-1 (IL-1).
PRK (proliferation-related kinase) is a serum/cytokine inducible STK that is involved in regulation of the cell cycle and cell proliferation in human megakaroytic cells (Li, B. et al. (1996) J. Biol. Chem. 271:19402–8). PRK is related to the polo (derived from humans polo gene) family of STKs implicated in cell division. PRK is downregulated in lung tumor tissue and may be a proto-oncogene whose deregulated expression in normal tissue leads to oncogenic transformation. Altered MAP kinase expression is implicated in a variety of disease conditions including cancer, inflammation, immune disorders, and disorders affecting growth and development.
The cyclin-dependent protein kinases (CDKs) are another group of STKs that control the progression of cells through the cell cycle. Cyclins are small regulatory proteins that act by binding to and activating CDKs that then trigger various phases of the cell cycle by phosphorylating and activating selected proteins involved in the mitotic process. CDKs are unique in that they require multiple inputs to become activated. In addition to the binding of cyclin, CDK activation requires the phosphorylation of a specific threonine residue and the dephosphorylation of a specific tyrosine residue.
Protein tyrosine kinases, PTKs, specifically phosphorylate tyrosine residues on their target proteins and may be divided into transmembrane, receptor PTKs and nontransmembrane, non-receptor PTKs. Transmembrane protein-tyrosine kinases are receptors for most growth factors. Binding of growth factor to the receptor activates the transfer of a phosphate group from ATP to selected tyrosine side chains of the receptor and other specific proteins. Growth factors (GF) associated with receptor PTKs include; epidermal GF, platelet-derived GF, fibroblast GF, hepatocyte GF, insulin and insulin-like GFs, nerve GF, vascular endothelial GF, and macrophage colony stimulating factor.
Non-receptor PTKs lack transmembrane regions and, instead, form complexes with the intracellular regions of cell surface receptors. Such receptors that function through non-receptor PTKs include those for cytokines, hormones (growth hormone and prolactin) and antigen-specific receptors on T and B lymphocytes.
Many of these PTKs were first identified as the products of mutant oncogenes in cancer cells where their activation was no longer subject to normal cellular controls. In fact, about one third of the known oncogenes encode PTKs, and it is well known that cellular transformation (oncogenesis) is often accompanied by increased tyrosine phosphorylation activity (Carbonneau H and Tonks N K (1992) Annu. Rev. Cell. Biol. 8:463–93). Regulation of PTK activity may therefore be an important strategy in controlling some types of cancer.
The phosphorylation of proteins on serine/threonine and/or tyrosine residues is now well established as a principal mechanism in the control of many cellular functions, such as the cell cycle, differentiation and signal transduction. Serine/threonine protein kinases add phosphate moiety to a serine or threonine residue of the substrate. Protein kinase substrates include elements of signal transduction pathway such as transcription factors or ion channels, as well as structural proteins such as filaments and cellular motors. Family of protein kinases is one of the largest in the genome. Classification of kinases is based on their sequence, tissue localization and domain topology.
Primary structures of kinases are rather conserved; functional prediction based on the sequence alone is difficult. A number of soluble and transmembrane proteins contain kinase domains along with other structural components; these multi-domain proteins are often referred to as kinases. Because of this additional information, multi-domain kinases are easier to classify. Tissue specific expression of kinases is often defined by transcription regulatory elements.
Serine- and arginine-rich (SR) proteins were originally characterized by a shared epitope that cross-reacts with the monoclonal antibody mAb104, at least one N-terminal RNA recognition motif and a basic C-terminal domain rich in serine and arginine residues, often arranged in tandem repeats. Since that time, many more potential SR proteins or SR-like proteins have been identified using different monoclonal antibodies, so that the number of such proteins may well reach 80 or more. There is evidence that the SR domain is involved in protein-protein interactions as well as protein-RNA interactions, and may serve as a localization signal directing proteins to nuclear speckles
The kinase of the present invention is similar to Serine- and arginine-rich (SR) or CDC like kinase 4, which is involved in cell cycle control. CDC like kinase 4 is one of the serine-arginine-rich (SR) protein kinases. The substrates of these enzymes include the components of splicing machinery. By changing their substrates' phosphorylation states, they affect these proteins' activities and intracellular distribution. As a result of modification, their substrates may select different splice sites, which results in alternative splice variants. CLK and SRPK are the two notable examples of SR kinases. CLK is involved in regulation of splicing, like most of SR kinases, while SRPK phosphorylates protamine, a small basic arginine-rich protein that replaces histamine in developing spermatozoa. Unlike other SR kinases, SRPK is localized in cytoplasm and expressed predominantly in germ cells.
The human CDC like kinase 4 gene of the present invention can be expressed in yeast to identify possible interactors; this can be done by means of a complementation assay or a two-hybrid experiment. Artificially synthesized enzyme as well as derived peptides can be used to activate or inhibit cellular processes modulated by this kinase. Immunoassay or PCR may be used to measure the concentration of this protein and detect abnormally developing tissue or cancerous growth.
For a review of serine-arginine-rich (SR) protein kinases and CDC like kinase 4, see the references of Nayler et al. Biochem J Sep. 15, 1997; 326 (Pt 3):693–700, Colwill et al., EMBO J. Jan. 15, 1996; 15(2):265–75, Papoutsopoulou et al., Nucleic Acids Res Jul. 15, 1999; 27(14):2972 80.
Kinase proteins, particularly members of the serine-arginine-rich protein kinase subfamily are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of this subfamily of kinase proteins. The present invention advances the state of the art by providing previously unidentified human kinase proteins that have homology to members of the serine-arginine-rich protein kinase subfamily.